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From "Maysam Yabandeh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-1955) Reducer Preemption is too agressive
Date Thu, 17 Apr 2014 17:11:18 GMT

    [ https://issues.apache.org/jira/browse/YARN-1955?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13973150#comment-13973150
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Maysam Yabandeh commented on YARN-1955:
---------------------------------------

I am thinking of a patch that postpones triggering reduce preemption until a certain period
from scheduling of the mapper is passed. I.e., add a timestamp to each scheduled mapper.

I highly appreciate the feedback.

> Reducer Preemption is too agressive
> -----------------------------------
>
>                 Key: YARN-1955
>                 URL: https://issues.apache.org/jira/browse/YARN-1955
>             Project: Hadoop YARN
>          Issue Type: Bug
>            Reporter: Maysam Yabandeh
>            Assignee: Maysam Yabandeh
>
> We observed cases where the reducer preemption makes the job finish much later, and the
preemption does not seem to be necessary since after preemption both the preempted reducer
and the mapper are assigned immediately--meaning that there was already enough space for the
mapper.
> The logic for triggering preemption is at RMContainerAllocator::preemptReducesIfNeeded
> The preemption is triggered if the following is true:
> {code}
> headroom +  am * |m| + pr * |r| < mapResourceRequest
> {code} 
> where am: number of assigned mappers, |m| is mapper size, pr is number of reducers being
preempted, and |r| is the reducer size.
> The original idea apparently was that if headroom is not big enough for the new mapper
requests, reducers should be preempted. This would work if the job is alone in the cluster.
Once we have queues, the headroom calculation becomes more complicated and it would require
a separate headroom calculation per queue/job.
> So, as a result headroom variable is kind of given up currently: *headroom is always
set to 0* What this implies to the speculation is that speculation becomes very aggressive,
not considering whether there is enough space for the mappers or not.



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